1. Field of the Invention
The embodiments discussed herein are related to a semiconductor apparatus and a method of fabricating the semiconductor apparatus.
2. Description of the Related Art
Generally, for a power semiconductor apparatus, surface electrodes formed on a surface of a semiconductor apparatus are connected by the wire bonding technique using aluminum wires. However, recently, the wire bonding technique has been replaced with a packaging technique that solders to the surface electrode a lead frame for electric wiring, an external electrode terminal or the like (hereinafter “bonding conductor”).
A semiconductor apparatus has a problem in that when high current flows, the semiconductor apparatus generates a large amount of heat. Therefore, the bonding conductor works as a heat dissipater for releasing the heat and enhancing cooling efficiency and works as an interconnection for electrically connecting the semiconductor apparatus. Heat dissipation efficiency is ensured by giving a certain volume to the bonding conductor. For this reason, the area on the semiconductor apparatus for soldering the bonding conductor needs be large relative to the bonding conductor.
In such a semiconductor apparatus, on an aluminum (Al) electrode formed on the semiconductor apparatus, a nickel (Ni) layer and a gold (Au) layer are formed in this order, enabling the bonding conductor to be soldered on the aluminum electrode.
As a method for forming such nickel and gold layers, the following method has been proposed. In a semiconductor chip where a back electrode is connected to a circuit pattern on an insulating substrate and a front electrode is connected to a bonding conductor, an electrode film consisting of two layers (a nickel (Ni) layer and a gold (Au) layer deposited on the nickel layer) is formed, by an electroless plating method, on an aluminum (Al) layer forming the front electrode (see for example Japanese Laid-Open Patent Application No. 2005-051084).
Another method includes, in order to nickel-plate an aluminum electrode of a device as barrier metal or as a protruding electrode, a photo etching step for etching the device by acidic liquid or alkaline liquid, a zincate process step for performing a zincate process using an alkaline zincate solution, an activation step for activating a surface of the aluminum electrode by immersion of the aluminum electrode in an alkaline solution having a reducing agent, and an electroless nickel plating step for immersing the aluminum electrode in an electroless nickel plating solution of a redox reaction type with the reducing agent solution adhered to the aluminum electrode. As an electroless plating solution, an electroless Ni—P plating solution or an electroless Ni—B (boron) plating solution can be used. An alkaline plating solution using sodium hypophosphite as a reducing agent can also be used. As an electroless gold plating solution, generally, metal salt made from gold cyanide salt such as potassium dicyanoaurate(I) is used with potassium borohydride or dimethylamine-borane (DMAB) being a reducing agent (see Japanese Laid-Open Patent Application No. H11-214421).
Through extensive research, the inventors have newly identified the following. If electroless nickel plating is performed using an electroless Ni—P plating solution having sodium hypophosphite as a reducing agent, such as the example given in Japanese Laid-Open Patent Application No. H11-214421, sodium (Na) in the electroless Ni—P plating solution remains in the nickel plating layer and at the interface between the nickel plating layer and the aluminum electrode. This sodium passes through the aluminum electrode and diffuses into the semiconductor substrate consequent to, for example, heat treatment such as soldering during the mounting of the fabricated semiconductor chip. As a result, the threshold voltage (Vth) characteristics of the semiconductor apparatus degrades.